If you need to buy dev. kits, Arrow shows that they have 41 at $28.71 and Digikey now has 871 in stock as well for $33.60.

You can't see them if you follow the link provided for the related ST.com website, but if you go to Digikey .com dirrectly and search for STM32F429I-DISCO, then you can see the dev boards stock available.